Attenuators are circuits used to control amplitude of a signal either continuously or in steps. When the signal is controlled in steps by an attenuator, the attenuator is referred as a step attenuator. The attenuators or step attenuators are widely employed in various electronic devices, e.g. radio frequency transceivers or radio frequency frontends in wireless communication devices including, for example user equipment or mobile devices and base stations, multi-antenna systems in radio base stations for both communication and localization, as well as in other general electronic circuits and equipment, such as automatic gain control circuits and measurement equipment etc. For radio frequency frontends, an on-chip step attenuator designed in Complementary Metal-Oxide-Semiconductor (CMOS) technology has advantages of small size, low cost, flexible and high integration level etc. There are some requirements for designing on-chip step attenuators, such as good linearity, low insertion loss, wide bandwidth and accuracy in attenuation steps etc.
In Cheng, W. et al, A Wideband IM3 Cancellation Technique for CMOS Attenuators, IEEE International Solid-State Circuits Conference, 2012, and in Cheng, W. et al, A Wideband IM3 Cancellation Technique for CMOS TT- and T-Attenuators, IEEE Journal of Solid-State Circuits, 2013, Vol. 48, NO. 2, on-chip Pi-type and T-type step attenuators are disclosed which provide attenuation steps of 6, 12, 18 and 24 dB. However, the disclosed Pi-type and T-type step attenuators have large insertion loss when working at lower attenuation mode, i.e. when attenuation level is lower, such as attenuation steps of 6 or 12 dB, especially when no attenuation is needed. High insertion loss will reduce gain and required signal to noise ratio (SNR) for input signals. Further, the disclosed Pi-type and T-type step attenuators also suffer from switch leakages which damage attenuation levels for the input signals at high frequencies during deep attenuation mode.
In Xiao, J. et al, A High Dynamic Range CMOS Variable Gain Amplifier for Mobile DTV Tuner, IEEE Journal of Solid-State Circuits, 2007, Vol. 42, No. 2, a variable gain amplifier suitable for mobile digital television (DTV) tuners is presented. Variable gain is achieved by using a capacitive attenuator and current-steering transconductance stages. Although the presented variable gain amplifier provides gain and attenuation, its linearity is poor due to active devices, i.e. the transconductance stages. A poor linearity will result in poor frequency selectivity for a radio frequency front-end, and thus degrade required SNR due to interferences from other unwanted frequency signals.